In recent years, there has been a demand for the reduction of carbon dioxide generated in various fields to reduce global warming. For example, in the automobile field, an increasing number of gasoline vehicles of the related art are being shifted to electric vehicles or hybrid vehicles equipped with a secondary battery exhausting a small amount of carbon dioxide, and particularly, the development of a lithium ion secondary battery having an influence on travel distance, safety, and reliability has been attracting attention. The above-described lithium ion secondary battery is, generally, made up of a non-aqueous electrolytic solution, a separator, an external packaging material, and the like in addition to a cathode including a cathode active material layer formed on a cathode current collector and an anode including an anode active material layer formed on an anode current collector.
In the past, an oxide of a transition metal including lithium was used as a cathode active material for the ordinarily-distributed lithium ion secondary battery, and the cathode active material was formed on an aluminum foil that served as a cathode current collector. In addition, a carbon material such as graphite was used as an anode active material, and the anode active material was formed on a copper foil that served as an anode current collector. In addition, the cathode and the anode were disposed through a separator in an electrolytic solution containing a non-aqueous organic solvent in which a lithium salt electrolyte was dissolved.
The lithium ion secondary battery is charged and discharged as described below. During the charge, lithium ions held in the cathode active material are de-intercalated and thus discharged into the electrolytic solution, and in the anode active material, the lithium ions from the electrolytic solution are absorbed between the crystal layers of the carbon material, thereby developing a reaction. In addition, during the discharge, a reverse reaction of the reaction during the charge progresses, the lithium ions are released from the anode active material, and are occluded in the cathode active material, thereby developing a reaction.
However, in a system in which a carbon material such as graphite is used for the anode, when the system is almost fully discharged, the anode potential reaches near 0 V, and therefore dendrite is precipitated. As a result, lithium ions that were originally intended to be used for electron transportation are consumed, and furthermore, the anode current collector is corroded and deteriorated. When the above-described corrosion and deterioration proceeds, there is a possibility of the characteristic deterioration or malfunction of the lithium ion secondary battery. Therefore, in the system in which a carbon material such as graphite is used for the anode, the precise control of the charge and discharge voltage is required. In the above-described system, while the potential difference between the cathode active material and the anode active material is, theoretically, high, only a part of lithium ions can be used, and there is a problem with the charge and discharge efficiency.
As a result, in recent years, research and development of an anode active material capable of obtaining a particularly high potential have been actively conducted. For example, since titanium dioxide has a potential of approximately 1.5 V, which is higher than the potential of a carbon material of the related art, titanium dioxide has attracted attention as a material that does not precipitate dendrite, is extremely safe, and is capable of obtaining favorable characteristics.
For example, PTL 1 describes anatase-type titanium dioxide and rutile-type titanium dioxide as examples regarding a secondary battery in which a titanium oxide that is obtained by spraying and drying a slurry containing a water-containing titanium oxide, and heating and removing an organic binder, and has a pore amount of secondary particles in a range of 0.005 cm3/g to 1.0 cm3/g is used as an electrode active material.
In addition, recently, there has been a report saying that titanium dioxide having a bronze-type crystal structure is also a promising anode active material. For example, PTL 2 describes a secondary battery in which bronze-type titanium dioxide having a micron-size isotropic shape is used as an electrode active material.
Additionally, NPL 1 describes battery characteristics in a case in which brookite-type titanium dioxide is used as an anode active material.
Here, as crystalline titanium dioxide, for example, an anatase-type crystal phase, a rutile-type crystal phase, a brookite-type crystal phase, a bronze-type crystal phase, a hollandite-type crystal phase, a ramsdellite-type crystal phase, and the like are known. However, a number of studies have been thus far conducted regarding sole crystal phases, but there have been few studies regarding the battery characteristics in a system containing a mixed crystal phase or an amorphous phase. Particularly, thus far, there have been no actual reports regarding titanium dioxide satisfying electric capacitance, cycle characteristics, and high-rate charge and discharge characteristics.
For example, a secondary battery in which the anatase-type or rutile-type titanium oxide described in PTL 1 is used as an anode active material has favorable cycle characteristics, but has a small electric capacitance of 160 mAh/g. Based on the above-described fact, to obtain a predetermined battery capacitance, it is necessary to use a large amount of the anode active material. Therefore, in the secondary battery described in PTL 1, there is a problem in that the weight or volume of the entire battery increases.
In addition, the secondary battery in which the bronze-type titanium dioxide as described in PTL 2 is used as an electrode active material has a small electric capacitance of 170 mAh/g, and furthermore, the process therefor is complicated and requires a long period of time, and therefore there are a number of problems in putting the secondary battery into practical use. In addition, PTL 2 describes nothing about the cycle characteristics or the high-rate charge and discharge characteristics.
Meanwhile, NPL 1 describes the battery characteristics in a case in which brookite-type titanium dioxide is used as an anode active material. However, in the technique described in NPL 1, the initial electric capacitance is high, but the electric capacitance decreases to 170 mAh/g after 40 cycles, and therefore there is a problem in that the cycle characteristics are poor. In addition, in NPL 1, since the charge and discharge rate is as low as C/10 (0.1 C), it is evident that, in a case in which high-rate charge and discharge is carried out, the cycle characteristics further deteriorate.